Ground-Controlled Interception

Ground-Controlled Interception

One of the most visually striking features on the 100-acre Calumet Air Force Station property is a set of nine towering metal structures hidden in the woods on the north end of the property. These towers - along with an identical set to the southwest - served as the antennas for the Ground Air Transmit Receive (GATR) control sites. These GATR sites were the radio link from the ground-based radars and direction centers of the SAGE network to the airborne interceptor aircraft operating in their sector. Working together, they allowed the Air Force to implement ground-controlled interception (GCI) tactics where radar personnel could identify threats, plot intercept courses, and guide the aircraft to their targets.

Ground-Controlled Interception

The idea of ground-based radars or observers guiding interceptor aircraft to targets was not a new concept during the Cold War. This air defense tactic had existed in one form or another since bomber aircraft were first used in WWI. During the Second World War, the British pioneered the first large scale ground-controlled interception network referred to as the Dowding System. It combined data from stationary, non-rotational radars looking out over the coast along with manned observer stations of the Royal Observer Corps to track incoming German aircraft and plot their likely courses. The system used a strict hierarchal approach where data from the radar and observer stations was fed to the Fighter Command Headquarters where it was organized and plotted out on scale battle maps. Specific information was then passed down to the relevant Group headquarters, where interception tactics were coordinated. From there, orders were passed to the specific Section headquarters, who would scramble the fighters for the actual intercept. By 1940, the British realized the manned observer posts provided inadequate coverage, especially during nighttime bombing raids. Additionally, the complex process of pinpointing a target aircraft using data from multiple fixed radar stations was time consuming and inherently inaccurate. This led to the development of the first rotational radar, the AMES Type 7. This allowed a single radar station to provide a complete 360 degree view of the airspace around the station with the exact position of targets quickly available to the operator, a concept known as a Plan Position Indicator (PPI). This is the same type of radar display still in use today in everything from air traffic control to weather radars.

Early Cold War

With the end of WWII and the increasing tensions with the Soviet Union, the United States saw the need to implement their own large-scale early warning radar network and GCI plans. By 1950, the Defense Department had constructed 44 radar stations in the continental United States as part of the Lashup Radar Network. With the implementation of the Permanent Radar System (P System) in the 1950s, that number grew to 187 radar stations across the United States and Canada by 1957. These stations were manned by US Air Force Aircraft Control & Warning (AC&W) squadrons (and their Canadian counterparts) tasked with identifying and tracking aircraft in their designated air defense sectors. Much like the British Dowding System of WWII, the system relied on a hierarchal approach, although the technology made the system much more efficient. Data from the radar stations was fed to a Manual Air Defense Control Center, where the data was combined with all radar stations in the sector and plotted out by hand on large plexiglass screens. The radar data was passed on from the sector control center up to Air Defense Command (ADC) in Colorado, where they maintained a unified picture of the entire continent's air defense data and had overall control of all intercept orders. Intercept paths would need to be manually calculated, and instructions would be radioed to the interceptor pilots.


Diagram of the SAGE network and its components

Much like the British system in WWII, this entire process was relatively slow and required extensive amounts of manpower to operate. With the large number of radar stations already in use, and numerous more planned in the Mid-Canada Line and DEW Line, it became completely impractical to process and combine all of the radar data manually. The only solution would be to use computers to help handle the data from all of the radar stations. In 1957, work began on the Semi-Automatic Ground Environment (SAGE), an enormous and complex system of advanced computers capable of processing the radar data and air defense capabilities for all of North America's airspace. Radar data including the distance and height of the target, the radar antenna's position, and Identification Friend or Foe (IFF) data would all be processed at each radar station by an AN/FST-2 Coordinate Data Transmitting Set, which would then pass the data through the phone lines to a SAGE Direction Center. There were a total of 23 Direction Centers, each with control over their own Air Defense Sector. Here, the data would be received and fed into the enormous AN/FSQ-7 Combat Direction Central computer. The FSQ-7 was the largest discrete computer system ever built, weighing over 250 tons and containing 60,000 vacuum tubes. It was capable of processing the data from all of the radar sites within the sector and displaying it as a unified picture for the radar operators. The computer would display available intercept options within range of any targets (including fighter aircraft and Bomarc missiles) and the Weapons Controller could dispatch them directly from their computer console. The computer was capable of calculating the estimated trajectory of the target aircraft and the necessary intercept path for the aircraft or missile. This intercept data would then be sent via phoneline back to the radio transmitter and sent to the aircraft from there. In the event that a Direction Center was destroyed or out of commission, certain radar stations were designated as Backup Intercept and Control (BUIC) sites and could assume control over intercept operation for the entire sector.

American and Canadian personnel working side by side in the SAGE Direction Center at K. I. Sawyer AFB

While the SAGE Direction Center was responsible for monitoring the airspace with its sector and coordinating intercepts with any possible threats, it would also pass the data up to a SAGE Combat Center. The Combat Center was responsible for supervising all of the activity within multiple air defense sectors, and was capable of taking full control over the entire nation's air defense if required. In turn, each Combat Center would also pass their data up to the NORAD Command Center in Colorado, which oversaw all of the airspace for North America.


Antenna towers at the GATR receiver site

When a possible threat was detected and fighter aircraft were scrambled to intercept a target, the data would need to be relayed to the aircraft. Whether this was a manually plotted intercept path in the pre-SAGE days or a set of computer generated instructions, it would be transmitted to the aircraft by one of the numerous Ground Air Transmit Receive (GATR) sites around the country. These sites were usually located on or near one of the radar stations, much like the one at the Calumet Air Force Station. The GATR setup at CAFS consisted of two separate antenna sites, one for the transmitter equipment and one for the receiver equipment. The receiver site was to the north of the main cantonment area of the station, directly past the motorpool. The transmitter site was to the west of the station, past the access road to the operations area at the top of the mountain. Each site had a cement block building that housed the radios, signal amplifiers, and other related equipment along with basic facilities and infrastructure like HVAC and restrooms. The sites were far enough from the main area that they could not use the steam heat from the main heating plant and instead had their own heating. These were originally coal-fed boilers and were later upgraded to fuel oil. Likewise, they had their own septic tanks and drain fields instead of using the main wastewater system. Both sites were also surrounded by their own perimeter fences in addition to radar station's main perimeter fence with their own gates for access control. The receiver site had a total of nine antenna towers, and the transmitter site had twelve. There was also a microwave relay antenna and two omni-directional antennas located at the transmitter site.

Voice Communications

Prior to the implementation of the SAGE system, ground controllers would need to manually plot out both the flight path of the target aircraft and the intercept path for the responding fighter aircraft. They would then need to relay these instructions to the pilot via radio. The standard radio equipment for most GATR sites consisted of single-channel radio transmitters and receivers each programmed to a specific frequency in either the ultra-high frequency (UHF) or very-high frequency (VHF) bands. Some UHF examples include the AN/GRT-3 and AN/GRT-22 transmitters, and AN/GRR-7 and AN/GRR-24 receivers. The VHF variants were the AN/GRT-21 transmitter and AN/GRR-23 receiver. Since each device would only be used for a single frequency, they would need racks and racks of radios for all of the frequencies that would be in use. The radios would then go through multi-couplers so that multiple radios could use the same antenna. There were also some multi-channel transceivers (combined transmitter and receiver) such as the AN/GRC-27 and AN/GRC-171 (UHF) and AN/GRC-211 (VHF) that were used, but the stories we've heard said these were in much smaller numbers compared to the single channel radios and in some cases simply used as backup radios.

Ground to Air Datalink

The SAGE system revolutionized the tactics of ground-controlled interception. Data could be compiled from multiple interconnected radar stations in near real-time to provide accurate positioning data of all aircraft in the air defense sector. Instead of manually plotting out the flight paths and intercept coordinates by hand, radar operators could use the computer to calculate all of this for them. On April 1, 1960 the Calumet Air Force Station officially joined the SAGE network as part of the Sault Ste. Marie Air Defense Sector. The sector's SAGE Direction Center was located at K. I. Sawyer Air Force Base near Gwinn, MI.

F-106 Delta Dart, the first aircraft equipped with the TDDL guidance system

An additional benefit of the SAGE system was that it allowed the personnel working in the Direction Centers to directly dispatch and control weapons systems to intercept enemy aircraft. Starting around 1960, work began on a ground to air datalink system that would allow intercept data to be sent directly to the autopilot of fighter aircraft from the SAGE computers. This means a weapons operator could identify a target, order an intercept, and automatically guide the jets to the correct intercept coordinates. The only thing the pilot would need to do was takeoff, perform the actual intercept (identify and engage the target if necessary) and land the aircraft again when it returned to base. The system used an AN/GKA-5 Time Division Data Link (TDDL) to decode the guidance instructions and an AN/FRT-49 Electronic Guidance Signals Transmitting Set to transmit them to the interceptor aircraft. Typically, these aircraft would have been F-106 Delta Darts equipped with either AN/ARR-60 or AN/ARR-61 airborne radio receivers, which were part of the onboard MA-1 Fire Control System. In addition to the F-106 fighters, this same system was also linked in to the CIM-10 Bomarc surface-to-air missile sites, such as the one at the Kincheloe AFB BOMARC Site at the eastern end of the Upper Peninsula. This entire system was often simply referred to as TDDL. In April of 1961, the Sault Ste. Marie ADS became the first air defense sector to be fully equipped with a TDDL system. This meant a radar operator at either K. I. Sawyer AFB or Calumet Air Force Station (since it was a backup interceptor site) could identify a target and scramble fighters and guide them or launch missiles at the target simply with the press of a button on their radar console.

Status of the GATR buildings

As the radio technology improved and the equipment took up less and less space, the two GATR sites were combined into one. Both the transmitter and receiver radios were all moved to the transmitter building on the west end of the property. The exact date of this is unclear, but we know that by 1985 the former receiver building had been repurposed as a childcare center for the on-base family housing. The radio equipment room had been turned into a game room, a small kitchen was added, and a playground was installed outside underneath the antenna towers.

Blueprints from 1985 showing the repurposed receiver building - now a childcare center.

When the station officially closed down in 1988, the equipment was all removed but the antenna towers at both sites were left intact. It is unclear if the buildings were repurposed when the Keweenaw Academy moved in to the facility a decade later, but we've heard that supposedly they used the transmitter site as a storage building and the receiver building may have served as some sort of break room or rec center. The inside has been trashed by vandals, but there are still the remains of a piano, vending machine, and couch inside. The building is one of the stops on our walking tours, and standing under the radio towers provides a really cool photo opportunity!

When the property was auctioned off by the county back in 2021, the transmitter site was still owned by the federal government. It was auctioned off separately and was not included in our purchase of the Calumet Air Force Station. The towers are still visible through the trees though.

Sources and further reading